The present invention relates to a present position locating method, a positioning device, and an electronic instrument.
The global positioning system (GPS) is widely known as a satellite positioning system, and is utilized for a car navigation system and the like. In the GPS, GPS satellite signals are respectively transmitted from a plurality of GPS satellites which orbit the earth, and a GPS receiver calculates (locates) its present position based on the received GPS satellite signals.
The GPS receiver generally calculates its position as follows. Specifically, the GPS receiver generates satellite sets (i.e., combinations of four or more GPS satellites) based on the acquired GPS satellite signals, and performs positioning calculations corresponding to each satellite set to calculate the present position of the GPS receiver. The GPS receiver selects the present position considered to have the highest accuracy from the present positions calculated corresponding to the satellite sets based on an index such as a position dilution of precision (PDOP), and determines the selected present position to be the present positioning result.
When calculating the present position corresponding to each satellite set, an estimated positioning accuracy decrease rate may be calculated corresponding to each satellite set before positioning. A satellite set considered to have the highest positioning accuracy (i.e., the lowest estimated positioning accuracy decrease rate) may be initially acquired based on the estimated positioning accuracy decrease rate to locate a position. The term “estimated positioning accuracy decrease rate” refers to an index specified by the constellation of GPS satellites. The estimated positioning accuracy decrease rate decreases as the extent of the constellation of GPS satellites increases. This is because the positioning accuracy tends to increase as the extent of the constellation of GPS satellites increases. This method is considered to be effective for accurately locating a position when a long period of time has elapsed after the preceding positioning (see JP-A-2006-90839, for example).
A method which calculates the present position by performing positioning calculations corresponding to each of a plurality of GPS satellite sets takes time for positioning. For example, the period of time required for positioning increases in proportion to the number of satellite sets. In recent years, a portable electronic instrument (e.g., portable telephone or wristwatch) has been provided with a GPS function. Since a portable GPS receiver is normally utilized during travel, the GPS receiver must locate its present position within a predetermined time limit (e.g., at intervals of one second).